1.Field of the Invention
The present invention relates generally to a detection device for a spectrophotometer system, and particularly to a light detection device with an analogue input and a digital output.
2. Description of the Prior Art
In a spectrophotometer system the concentration of a substance in a test sample is measured. Such a system generally comprises a continuous light source, optical elements and a detection device for detecting a test sample beam of light and/or a reference beam of light. The advantage of detecting both a test and a reference beam of light, i.e. with a two channel system, is that the measurement of the concentration of the substance is made independently of the variations of the light source. In a two channel system the test sample beam first enters a flow cell, through which test samples are passed from a chromatographic column, and then impinges on a first detector with an intensity I and the reference beam impinges directly, via optical elements, on a second detector with an intensity I0.
The detector output signals are then processed in the detection device to firstly calculate the absorbency of light in the test sample in accordance with the well known equation,
A=xe2x88x92log (I/I0)
Thereafter the concentration in the test sample is calculated by using the following equation,
A=∈*1*C
where 1 is the path length of the flow cell, C is the concentration and ∈ is the absorbability.
U.S. Pat. No. 4,678,917 describes a multichannel spectrophotometer comprising an array of photo detectors for obtaining data for a wide spectrum chromatogram. The signals output by the photo detectors enter sample and hold circuits before they, via a multiplexor switch, are converted by an A/D-converter. The radiation source is a deuterium lamp sending out continuous radiation through a sample cell. The object of this invention is to provide a simultaneous measurement for a wide spectrum light, rather than for only one wavelength.
U.S. Pat. No. 4,318,618 describes an apparatus for automatically measuring changing values of absorbency. The described system is a two channel system having one measuring and one reference photo detector. The signals coming from the photo detectors are digitised via I/V converts and A/D converts before they are processed. The object of this invention is to secure that the absorption cell in which the sample is contained is aligned in a measuring zone in order to provide an accurate measurement.
U.S. Pat. No. 5,387,979 shows another example of a two channel spectrophotometer system. This system uses a pulsating (modulated) source of radiation giving rise to an alternating current component. This reduces the measurement response time of the for changes in the concentration of the specific substance measured.
U.S. Pat. No. 4,549,809 shows a one channel photometric measurement method, wherein readings are taken at precisely measured intervals as a function of the width of the cuvette and at the same distance from the leading wall of each cuvette containing the test sample.
The detection devices for the spectrophotometers in the above described U.S. Pat. Nos. generally comprise detectors, such as photo cells, integrator means, amplifying means, analogue to digital (A/D) converters, buffering means and evaluation means, such as a microprocessor or computer.
The A/D converters that are used for such a detection device traditionally use external sample and hold circuits prior to the converter. This causes problems, firstly in that the lines between the sample and hold circuits are subject to interferences and thus the accuracy of the A/D conversion deteriorates. Secondly the sampling speed will be affected negatively by using external sample and hold circuits. This again will deteriorate the accuracy of the A/D conversion due to dark currents and charge injections.
The A/D converters in detection devices used for spectrophotometers are specially designed and therefore manufactured in small series, and hence expensive. The advantage of these specially designed A/D converters is that they have many options and can be adapted to all kinds of signals.
Since there are two signals that have to be converted in the detection device for the spectrophotometer, i.e. the reference signal and the test sample signal, two such A/D converters are required. Furthermore, these A/D converters require several additional control circuits to operate properly, due to all the available options mentioned above, and therefore require a large space. Thus, there is a need for an accurate A/D converter that is less space consuming and much cheaper in order to bring down the overall size and cost for the detection device.
The above mentioned U.S. Patents furthermore all comprise a radiation source sending out radiation continuously giving rise to dynamic output waveforms from the photo detectors, in which waveforms the peak values have to be determined in order to get an accurate measurement. The continuous radiation does furthermore not prescribe any high demands on the sampling frequency.
Accordingly, the object of the present invention is to provide a more accurate detection device, which requires substantially less space for accommodating the detection device in a spectrophotometer and also to reduce the manufacturing cost of such a device.
This object of the invention is solved by the device using a successive approximation A/D converter with an internal sample and hold circuit, which heretofore has been used in the audio industry, for example in DAT-tape recorders and other studio equipments. The use of such an A/D converter substantially reduces the space needed for the detection device, down to one third of the space required for a conventional detection device. The configuration of such an A/D converter with an internal sample and hold circuit enhances also the accuracy and the speed of the converter. Furthermore such an A/D converter is much cheaper, since it is mass produced.
The A/D converter according to the present invention is controlled by a sequence generator in order to secure proper functioning of the A/D converter in the detection device for the spectrophotometer.
To further improve the accuracy of the detection device the radiation source is a flash generating source giving rise to a step response from the photo detectors, of which step response only the start and end values need to be measured.